Heat leaks through every surface. The U-value tells how fast. U-value is the heat flow in watts through 1 m² for each 1 K of temperature difference (W/m²K). Lower U means better insulation.

Air leaking in and out also carries heat. We count it as air changes per hour (ACH). One ACH means the full room volume is replaced every hour.

• 0.33 = density × specific heat of air ÷ 3600, in Wh/m³K.
• n = air changes per hour; V = room volume in m³.

Worked example — 120 m² house in Chandigarh

Design outdoor 3 °C, indoor 21 °C, ΔT = 18 K. Ground below floor about 15 °C. Height 3 m, n = 1.0 ACH.

• Walls: 2.0 × 140 m² × 18 = 5,040 W
• Windows: 5.7 × 20 m² × 18 = 2,052 W
• Roof: 3.0 × 120 m² × 18 = 6,480 W
• Floor: 1.0 × 120 m² × 6 = 720 W
• Ventilation: 0.33 × 1.0 × 360 m³ × 18 = 2,138 W
• Total = 16,430 W ≈ 16.4 kW → with 10% margin, about 18 kW
• With 50 mm roof insulation (U = 0.6), roof loss drops to 1,296 W and total to about 11.2 kW.

Low flow temperatures raise COP. Underfloor heating works at 30–40 °C. Fan coils work at 40–50 °C. Old radiators may need 55–65 °C. Weather compensation lowers the flow temperature on milder days and saves energy.

Remember

• Roof losses dominate in Indian flat-roof houses.
• Insulate first, then size the heat pump.
• Use low flow temperatures: underfloor 30–40 °C is ideal.
• Design at the local winter design temperature, not the average.